RF communications systems typically communicate using at least one of three different modes of operation. The first mode, called simplex, is a one-way mode of operation, in which a transmitter from one location transmits data to a receiver at another location. For example, a broadcast radio station transmits data one-way to radios. The second mode, called half duplex, is a two-way mode of operation, in which a first transceiver communicates with a second transceiver; however, only one transceiver transmits at a time. Therefore, the transmitter and receiver in a transceiver do not operate simultaneously. For example, certain telemetry systems operate in a send-then-wait-for-reply manner. The third mode, called full duplex, is a simultaneous two-way mode of operation, in which a first transceiver communicates with a second transceiver, and both transceivers may transmit simultaneously; therefore, the transmitter and receiver in a transceiver must be capable of operating simultaneously. In a full duplex transceiver, signals from the transmitter must not interfere with signals received by the receiver; therefore, transmitted signals are at transmit frequencies that are different from received signals, which are at receive frequencies. The difference between a transmit frequency and a receive frequency is called the duplex frequency. For example, certain cellular telephone systems operate using a full duplex mode of operation.
Full duplex transceivers using a single antenna often use a duplexer to couple the transmitter and receiver to the single antenna. A duplexer enables simultaneous transmission and reception of RF signals by providing a transmit passband that does not overlap with a receive passband, which prevents interference between transmit and receive signals. The non-overlapping area is also known as a guard band. Some communications protocols, such as specific Universal Mobile Telecommunications System (UMTS) bands have guard bands that are narrow relative to the transmit and receive passbands; therefore, providing the required transmit and receive passbands with minimal insertion loss while providing required isolation between transmit and receive signals may be difficult.
Thus, there is a need for a duplexer that provides required transmit and receive passbands with minimal insertion loss while providing required isolation between transmit and receive signals.